After some thought on this issue of the retaining bolt could have been caused by a previous owner or yard.
If the bolt will not come off and is spinning even with a little pressure applied underneatht the bolt head, I then resorted to having to remove the head of the bolt so it would go thru the hole in the glass when removing the centerboard.
When one removes the centerboard bracket and then reapplies it with the retaining bolt, you have to be very careful to align the bolt threads into the stainless steel nut (welded onto the centerboard bracket). If not, the threads when rebolting will sieze up and then you will have to go thru removing the head of the bolt. I have seen where it was overtightened that even the weld of the nut to the centerboard bracket was broken.
Never drill an inspection hole as you will not see anything and going thru the hull is not suggested.
Now the problem is getting the bolt head removed. Dremel now has a new cutting disc but even though, you have very little room in the recessed area which to get a dremel into which at most will be at an angle. If going this route will take time but I always take suggest caution. I ended up purchasing a new large drill bit and then tapped the center of the bolt head starting with a smaller drill bit and then using the larger new drill bit to drill out the center of the bolt. At that point following the drilling, I would then use a to break off the remaining side walls.
In the case of Brian, I think that the nut welded to the Center Board Bracket may have failed allowing it to spin and that being the case, I would use a small vice grips to get the remaining bolt head walls off which would take longer.
Generally the cause is when rebolting and misaliging the bolt and nut threads causing it to sieze which is what I saw in the past.
crazy dave